(1) Field of the Invention
The present invention relates to a microscope system for observing a microstructure, such as a micro-organism or a cell, with desired magnifications.
(2) Description of Related Art
Conventionally, a microscope system including an objective lens and a focusing lens and referred to as an "infinity system" includes a microscope device in which a parallel ray portion is formed within a space between the objective lens and the focusing lens. This makes it possible to set a distance between the objective lens and the focusing lens at an arbitrary length to some degree. In such a type of microscope system, a focal length of the focusing lens is set from 160 mm to 250 mm in view of the total size of the microscope and ease of aberration correction. In general, a distance between a sample surface and an attachment plane of the objective lens, i.e., a parfocal length, is set at 45 mm. A diameter of a screw formed on a portion by which the objective lens is attached to a microscope body or, in other words, an outer diameter of a screw formed on an attachment portion of the objective lens, ranges from approximately 20 mm to 25 mm. This range is determined by the upper limitation on the size of a turret style magnification changer, referred to as a revolver, and the minimum limitation of a space for the parallel rays.
According to the conventional microscope described, the focal length of the focusing lens is set from 160 mm to 250 mm and the parfocal length of the objective lens is set at 45 mm.
A combination of the above parameters provides a distance between an object surface (a sample surface) and an end plane of the objective lens of about 50 mm. Such a distance causes many disadvantages when a multi-level change in a magnification factor of the microscope is performed. A composite magnification B of the objective lens and the focusing lens is given by EQU B=fI/F0
where F0 is the focal length of the objective lens and fI is the focal length of the focusing lens. Here, a focal length of the focusing lens is set at 200 mm and a magnification factor of the objective lens, for an extremely low magnification, is set at 1.times..
The above equation gives an objective lens with focal length of 200 mm.
A telephoto ratio of the objective lens must be set to 0.25 in this example in order for the objective lens having a focal length of 200 mm to be placed in a space having a size of about 50 mm (i.e., a space between the sample surface and the end plane of the objective lens). This makes it impossible to realize an objective lens which has a general telecentric characteristic at the object side and in which aberration is corrected.
The diameter of the screw on the attachment portion through which the objective lens is attached to the revolver ranges from 20 mm to 25 mm. When the objective lens is designed to offer a low magnification of 10.times. and a large numerical aperture N.A. of, for example, 0.5, a numerical aperture N.A.' at the image side of the objective lens (a "rear" numerical aperture) is set at 0.05.
In this case, a pupil diameter .phi.P of the objective lens is represented by .phi.P=2.times.N.A.'.times.fI. By letting the focal length of the focusing lens be 200 mm, the above equation results in a pupil diameter .phi.P=2.times.0.05.times.200 mm=20 mm. This reveals that it is necessary to place a lens having an effective diameter of 20 mm or more within the attachment portion by which the objective lens is attached to the revolver.
A demand for an objective lens having a large rear numerical aperture N.A.' is therefore present. In epi-fluorescence microscopy, which requires much more light for gaining a bright image, it is especially important to have the rear numerical aperture N.A.' of the objective lens be as high as possible. This is because the image brightness is determined by the rear numerical aperture N.A.' power 2.